1. Technical Field
The present invention relates to an inter-vehicle communication system and an inter-vehicle communication device that constitutes the inter-vehicle communication system. More particularly, the present invention relates to a technique to improve communication quality of inter-vehicle communication.
2. Related Art
Inter-vehicle communication is a dedicated short-range ad-hoc communication technique in which an inter-vehicle communication device mounted on a vehicle directly communicates with other inter-vehicle communication devices mounted on other vehicle without using a base station. In inter-vehicle communication, when the inter-vehicle communication device detects another inter-vehicle communication device in the surrounding area is performing data transmission, the inter-vehicle communication device suspends its data transmission so as not to interfere with the transmitted data.
However, in a situation where the communication device (hereinafter, referred to as “inter-vehicle communication device A”) fails to detect that other inter-vehicle communication device (hereinafter, referred to as “inter-vehicle communication device B”) is transmitting data due to an obstacle such as a building, the inter-vehicle communication device A may perform data transmission. In such a situation, since data from the inter-vehicle communication devices A and B interfere with each other, an inter-vehicle communication device C that is located at a position capable of receiving data from both the inter-vehicle communication devices A and B may fail to receive both data. This situation is also called a hidden terminal problem in the sense that the inter-vehicle communication device B is not detected by (is hidden from) the inter-vehicle communication device A.
A mobile station disclosed in JP-A-2008-17318 includes a communication unit configured to perform dedicated short-range communication and a communication unit configured to perform communication with a base station handling wide-area infrastructure communication, and notifies its location to the base station via wide-area communication. When the base station receives the notification, the base station assigns a dedicated short-range ad-hoc communication resource to the mobile station and transmits the assigned information to the mobile station. The mobile station performs dedicated short-range communication based on the assigned dedicated short-range ad-hoc communication resource. As a result, simultaneous data transmission by a plurality of mobile station using dedicated short-range communication is avoided.
According to JP-A-2008-17318, the base station assigns the dedicated short-range ad-hoc communication resource, and the mobile station needs to sequentially receive the assignment of the dedicated short-range ad-hoc communication resource assigned by the base station. This causes a problem in that the amount of data transmitted in downlink from the base station to the mobile station decreases.